Antenna pattern measurement method and device

ABSTRACT

This invention relates to a process for measuring an antenna diagram in which: 
     the antenna (30) is connected to a receiver (33) capable of supplying C/No measures (the ratio of the useful signal power to the noise power spectral density) and possibly phase variation measurements; 
     the antenna (30) is placed at the end of a mast (32) or a carrier satellite facing towards the sky to track the satellites in a constellation, the antenna reception band being included in the satellite antenna transmission band; 
     the C/No ratio and phase variations are automatically recorded for a given period; 
     the diagram for the said antenna is deduced by calculation.

TECHNICAL FIELD

This invention relates to a process and device for measuring an antenna diagram.

STATE OF PRIOR ART

Measurement methods are necessary to characterize antennas in order to:

use experimental methods to size antennas that cannot be modeled;

finalize the design of antennas that have already been designed by "approximate" models;

validate the results of modeling obtained with "exact" software, and use these models when there is a doubt about the physical meaning of some of the calculation results.

The purpose of the measurement equipment is to record the antenna radiation and matching characteristics:

by reproducing actual usage conditions (for example plane waves in reception) as accurately as possible, by using long external bases (far field) or compact bases (plane wave recreated at short distance in an anechoic chamber);

by precise readings of the near field (amplitude and phase) on a surface surrounding the antenna and by calculating the radiated far field;

by impedance measurements (antenna matching) that are obtained using a vectorial network analyzer, preferably in an anechoic chamber or in free space to prevent any coupling of the antenna with the measurement laboratory.

In a measurement base in far field like that shown in FIG. 1, the antenna to be measured 10 is placed in a direction determined with respect to a transmission antenna 11 perfectly calibrated in gain and in emitted power. The antenna to be measured, which may be protected by a radome, is connected to a device 12, for example measuring the power and the phase of the received signal. The distance D between the two antennas must be sufficient so that the waves received by the antenna to be measured are plane waves. A positioner 13 rotates the antenna 10 to be measured in different directions, for example in rotation φ and in sighting direction θ. The disadvantage of this type of measurement base is that it requires a large site, the size of which depends on the wave length. The length of this site may be several hundred meters.

In a compact base such as that shown in FIG. 2, which may be located inside a building, the waves emitted by an antenna 21 transmitting hemispheric waves are reflected by a reflector 22 that transforms the spherical waves into plane waves that are received by the antenna to be measured 23, which reduces technical effects due to distance. The disadvantage of this type of base is that uses accurate but expensive means, and furthermore which are more difficult to perfect.

In a near field base like that shown in FIG. 3, the signal power and phase in the near field emitted by a source 24 are measured, and antenna theory is applied in detail in order to obtain the characteristics of the antenna to be measured 25, using a measurement device 26. The disadvantage of this type of base is that very accurate and very precise measurement probes and sophisticated computer facilities are necessary.

In the space domain, the satellite localization/navigation sector is developing very quickly. The American G.P.S. (Global Positioning System) is an existing worldwide positioning system using a large number of satellites, that a navigator can use to determine his position and speed. The USSR has developed an equivalent system called GLONASS.

Other telecommunications satellite constellations with land mobile and/or navigation mobiles will be launched in the future, for example:

GLOBALSTAR operating with an S band down link;

IRIDIUM operating in the S band;

ORBCOMM operating in VHF;

STARSYS operating in VHF;

GNSS2 operating in the L band.

References [1] and [2] describe methods of measuring the attitude of a satellite using a GPS antenna.

Reference [3] describes how to use the signal/noise ratio for correction of errors due to multiple paths in GPS differential (or interferometric) phase measurements.

Reference [4] studies the use of GPS receivers and GPS/GLONASS receivers. This article illustrates the fact that this type of receiver is now in widespread use.

References [5] and [6] contain a technical description and characteristics of the global navigation system using the GLONASS-M satellite.

The purpose of this invention is to provide a means of making measurements of antenna diagrams, without the need for expensive measurement systems, making use of the satellites in a given constellation.

DESCRIPTION OF THE INVENTION

This invention proposes a process for measuring an antenna diagram in which:

The antenna is connected to a receiver capable of outputting measurements of the C/No ratio, which is the ratio of the useful signal power to the noise power spectral density. This receiver can also optionally measure the phase variation of the received signal carrier.

The antenna is placed at the top of a mast, or on a carrier satellite facing towards the sky to observe the satellites in a constellation, the antenna reception band containing the transmission band of the satellite antennas used.

The C/No ratios associated with observable satellites are automatically recorded for a given period.

The diagram for the said antenna is determined by calculation.

Advantageously, an antenna gain function can be calculated, making use of a limited number of coefficients. These coefficients are adjusted using measurements based on a least squares criterion.

For example, the antenna gain could be given by the following formula: ##EQU1## θ and φ are spherical coordinates; ai, bi, ci, di and ei are coefficients.

For example, the antenna may be in the L, S, C, VHF or UHF band.

For example, the satellite constellation may be of the GPS, GLONASS, GNSS2, GLOBALSTAR, IRIDIUM, ORBCOMM, STARSYS, SATIVOD or TELEDESIC type.

This invention also relates to a device for measuring an antenna diagram, characterized in that it comprises a mast, or a carrier satellite, at the end of which the antenna to be measured is placed, a receiver to receive signals from the antenna and a means for processing signals output from this receiver, and absorbent material placed on or around the mast.

Beneficially, the antenna is placed under a radome if protection is essential against the external media at all times. This is the case particularly if the antenna to be measured is already installed on a satellite or another sensitive carrier structure. A preamplifier calibrated as a noise factor is placed between the antenna and the receiver. The antenna may be placed on an azimuth and elevation positioner.

The process according to the invention has many advantages:

it does not require any special transmission equipment;

it can use low cost commercially available receivers such as GPS, GLONASS, GPS-GLONASS receivers;

the signal power and phase are no longer necessarily measured (which required a measurement of polarization losses), but instead the C/No ratio is measured that includes all losses that occur along the path. Therefore, a global measurement is made;

the diagram of an antenna for the reception of GPS or GLONASS signals (or signals from other constellations) mounted on a satellite already in orbit can be measured.

The equipment means to be used are much lighter in weight and less expensive than equipment necessary for conventional antenna bases (far field base, long base, near field base, compact base).

Antennas to be measured which are not specifically dedicated to reception from a satellite constellation must be relatively wide band.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1 to 3 illustrate different devices in prior art;

FIGS. 4 and 5 illustrate the device according to the invention;

FIG. 6 illustrates the gain curve of the antenna to be measured in terms of spherical coordinates;

FIG. 7 illustrates the set of GPS satellite plots for a period of 24 hours;

FIGS. 8 and 9 illustrate an example satellite pass and the corresponding C/No measurement curve;

FIG. 10 represents real C/No measurement curves corresponding to passes of several GPS satellites;

FIGS. 11 and 12 illustrate one characteristic of two alternative embodiments of the device according to the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

The invention can be used to measure an antenna diagram, for example in the L band, using satellites in a constellation and the associated receivers.

For example, the following satellite constellation types can be used:

GPS;

GLONASS.

In this case the frequency bands that can be used are:

for GPS:

L1→about 1575.42 MHz

L2→about 1227.60 MHz

for GLONASS: Different L1 and L2 frequencies are possible.

As shown in FIGS. 4 and 5, the invention consists of placing the antenna to be measured facing the sky, towards the various satellites 31 in a constellation. It is placed at the top of a mast 32 (or other support), in a location in which parasite reflections are minimized (no nearby buildings or obstacles, absorbent coating on the ground around the mast). The antenna to be measured is connected to a receiver 33, for example of the GPS or GLONASS or GPS-GLONASS (GNSS) type, possibly through a preamplifier 36 which may be built into the antenna.

The mast 32 is sized to be able to support the antenna 30. This antenna 30 may be of several types. For example, it may be a spiral or a parabola. It may be located under a protective radome.

The mast height must be a few wave lengths. In one example embodiment, the height of this mast is about 1 meter. It thus elevates the antenna above the nearest surfaces.

This mast must be made on a non metallic, dielectric material, for example Teflon (trademark).

Absorbent structures 34 placed on and around this mast may consist of foams containing carbon particles to absorb radio electric signals (for example made by the Emerson and Cumming company).

The receiver may be connected to a computer 35, which records the various data supplied by the equipment:

ephemeris tables for each of the satellites being tracked (optional);

measurements of the C/No ratio made for each of the satellites being tracked;

measures of the phase variation of the received signal carriers (optional).

If the receiver cannot acquire ephemeris tables for the satellites being tracked, it is assumed that they are obtained by the computer 35 connected to a database containing this information.

The gain Gr of the antenna to be measured is defined as a function of two angles, θ and φ (spherical coordinates). The representation of Gr is shown in FIG. 6.

It may be assumed that the gain of the antenna to be tested can be expressed as a limited series of elementary functions fi of θ and φ, with the coefficients ai, bi, ci, di and ei as parameters. ##EQU2##

The gain function Gr is then characterized by (n+1)×5 coefficients, in the case of this example.

The gain of the antenna is determined by estimating the coefficients ai, bi, ci, di and ei using the measurements of the ratio C/No and knowing angles θ and φ representing the sighting angles of the satellites being tracked, the C/No ratio being the ratio between the useful signal power to the noise power spectral density.

The angles θ and φ are known by reference to ephemeris tables:

φ=elevation=function (mast position, ephemeris)

θ=azimuth=function (mast position, ephemeris).

The equation of the link balance is obtained, where C/No is a ratio measured by the receiver ##EQU3## where: K=number of satellite being tracked

P_(ek) =power transmitted by satellite No. K

G_(ek) =gain of the satellite No. K transmission antenna

PIRE_(k) =associated intrinsic equivalent radiated power

D_(k) =distance between the antenna to be tested and satellite No. K

C=speed of light

f=working frequency for measuring the gain Gr

L.sub.(θ, φ) =various losses (polarization, atmospheric, rain, etc.) where L.sub.(θ, φ) >0 ##EQU4## T=equivalent noise temperature T_(ANT) =antenna noise temperature

L_(FRX) =losses in the feeder

T_(FRX) =temperature of the feeder

T_(R) =receiver noise temperature

T_(ANT), L_(FRX), T_(FRX) and T_(R) are assumed to be known.

When θ and φ are known (using ephemeris tables), then G_(ek), D_(k) and L are known; f is also known.

The unknown are P_(ek) and ai, bi, ci, di and ei.

In the case of a constellation with 24 satellites, K=1, 2, . . . , 23, 24.

Therefore the unknowns are:

P_(e1), P_(e2), . . . , P_(e24)

a₁, a₂, . . . , a_(n)

b₁, b₂, . . . , b_(n)

c₁, c₂, . . . , c_(n)

d₁, d₂, . . . , d_(n)

e₁, e₂, . . . , e_(n)

Therefore the unknowns represent the status vector of the gain estimating filter.

The process for estimating the gain will be an optimization process, therefore using the link balance as a model, and possibly using one of the following constraints given as an example:

*Continuity Constraint

G_(r) (θ, φ)-G_(r) (θ+Δθ, φ+Δφ) ≦ΔGr (Δθ, Δφ)

Δθ and Δφ are angular variation values

ΔG_(r) =maximum tolerable gain variation for the angular variations Δθ and Δφ.

*Symmetry of Revolution Constraint

G_(r) (θ₁, φ)=G_(r) (θ₂, φ) for all values of θ₁, θ₂, φ.

*Positive Gain Constraint

G_(r) (θ, φ)>0 for all values of θ and φ.

Therefore the process according to the invention consists of automatically recording C/No measurements over a long period, for example several days, using the receiver and an associated processing method, for example a computer.

In 24 hours, passes of the satellites in the constellation used will have the shape shown in FIG. 7. Note that plots of the satellites being tracked are fairly well distributed in all possible values for θ and φ. However, this distribution is not perfect. For a given antenna position, some ranges of values of θ and φ are never reached by satellites in the constellation used, due to the geometry of their orbits.

The C/No curve corresponding to the satellite pass shown in FIG. 8, is shown in FIG. 9 as an example.

The duration of a complete plot like that shown in FIG. 9 usually takes a few hours (time for one pass of a radionavigation satellite).

FIG. 10 shows real C/no measurement curves corresponding to several GPS satellite passes.

The rate of C/No measurements can be programmed by the chosen receiver.

The gain estimating software may typically be a least squares type optimizer, for which the criterion and constraints are given in the appendix.

The estimated values for ai, bi, ci, di, ei and P_(ek) are the values that minimize the residual energy, in the least squares sense of the term.

In one alternative embodiment, the constellations of GPS and GLONASS satellites are sized such that the same satellite is always located at the same "position" (θ, φ) in the sky at the same time of the day, within a few minutes.

Therefore, it would be possible to imagine the use of an azimuth and possibly elevation positioner according to prior art, using this method:

in azimuth: the horizontal plate 40 on which the antenna to be tested can rotate by a perfectly known programmable angle θ₀, as shown in FIG. 11;

in elevation: the plate 40 may be inclined by a programmable and perfectly known angle φ₀, as shown in FIG. 12.

Rotations of this assembly plate 40 used to support the antenna to be measured avoid the need to wait for too long before observing a significant change in the configuration of the satellite passes in the antenna coordinate system. Furthermore, this can minimize the lack of C/No measurements within some variation ranges of θ and φ, due to the geometry of the orbits of the satellites in the constellation used.

In one alternative of the invention, the antenna to be measured receives signals from geostationary satellites for which the orbital position, frequency and transmission gain are known. For example, the VHF, UHF, C, L, S and Ku reception bands can be used.

Therefore, the measurement principle described is applicable in this case, if the positioner described above is available.

In another alternative of the invention, it is possible to measure the diagram of an antenna for the reception of signals from constellations, in which the antenna is mounted on a satellite in orbit for which the attitude for the three axes is known perfectly.

Knowledge of this attitude can be used to determine the orientation of the antenna coordinate system for the antenna to be measured with respect to the observation directions of the satellites in the constellation used.

The problem with measuring the antenna diagram is therefore similar to the two previous cases. In this case, the antenna to be measured is connected to a constellation receiver onboard the carrier satellite.

Constellation signal receivers currently available can also observe detailed variations of the phase of received carriers (after spectrum despreading, when the signals used are transmitted in spread spectrum).

The phase of the received signal makes a skip equal to π (3.14159 . . . radians) when the observability direction of the satellite being tracked changes from the main lobe of the antenna diagram to one of the secondary lobes of the said diagram.

The set of (θ₀, φ₀) pairs associated with a phase change of π therefore forms the boundary between the main lobe and the secondary lobes of the antenna. Knowledge of this boundary, in addition to the measurements of the C/No ratio, can be used to refine the estimate of the diagram for the antenna to be measured.

The equation of this boundary H will be measured using measurements of the (θ₀, 100₀) pairs associated with the phase change, using a least squares criterion.

This equation is denoted:

    H(θ.sub.0, φ.sub.0)=0

The estimate of the boundary equation is denoted:

    H(θ.sub.0, φ.sub.0)=0.

The gain function of the antenna to be measured is assumed to be zero for all values of θ₀ and φ₀ satisfying this equation.

                  APPENDIX                                                         ______________________________________                                         Notation used                                                                  j =       satellite number in the observed                                               constellation, between 1 and j                                       n =       chronological order number of a measurement                                    made by the constellation receiver                                   N.sub.j min =                                                                            order number of the first measurement made                                     using satellite No. j                                                N.sub.j max =                                                                            order number of the last measurement made                                      using satellite No. j                                                 ##STR1## measurement of the C/No ratio made at time  n using satellite                  No. j                                                                k =       Boltzmann's constant 1.379.10.sup.-23 W/K.Hz                         L =       total losses (excluding free space losses)                                     until the input to signal processing                                           circuits in the constellation receiver                               T =       noise system temperature of the receiver                                       connected to the antenna                                             f.sub.L = signal frequency used, broadcast by the                                        constellation                                                        c =       speed of light = 3.10.sup.8 m/s                                      D.sub.j,n =                                                                              distance between the antenna and satellite                                     No. j at time n                                                      PIRE.sub.j =                                                                             intrinsic equivalent radiated power of                                         satellite No. j                                                      PIRE.sub.j =                                                                             estimate of the intrinsic equivalent                                           radiated power of satellite No. j                                    PIRE.sub.min =                                                                           minimum possible value of PIRE for a                                           satellite in the constellation                                       PIRE.sub.max =                                                                           maximum possible value of PIRE for a                                           satellite in the constellation                                       φ.sub.j.n =                                                                          sighting angle of satellite No. j, at time                                     n, with respect to the main center line of                                     the antenna diagram                                                  φ.sub.max =                                                                          maximum φ.sub.j.n angle considered                               θj,n =                                                                             angle of the direction of sight in the                                         antenna plane (defined by the x and y axes)                                    of satellite No. j, at time n, with respect                                    to the x axis                                                        Gr(θ,φ) =                                                                      gain function of the reception antenna                                         defined in the x, y, z coordinate system                             Gr(θ,φ) =                                                                      estimate for the antenna gain function                               i =       order of the coefficients for the antenna                                      gain function                                                        I =       maximum order of the coefficients of the                                       gain function                                                        f =       basic function for the antenna gain                                            function                                                             ai, bi, ci                                                                               of order i coefficients for the gain                                 di, ei =  function                                                             ai, bi, ci,                                                                              estimated order i coefficients for the gain                          di, ei =  function                                                             A =       vector of required unknowns to determine                                       the antenna gain and PIRE values                                     A =       solution vector containing parameters for                                      the gain function and PIRE values                                    Mj,n =    value of the measurement function                                    F(A; θ.sub.j,n ; φ.sub.j,n) =                                                  function of gain × PIRE                                        Useful equations                                                               Link balance                                                                   PIRE.sub.j = P.sub.ej × G.sub.ej                                          ##STR2##                                                                      Gain function                                                                  A = [PIRE.sub.1, . . . , PIRE.sub.j, ao, . . . , ai, bo, . . . , bi, co,       . . . , ci,                                                                    do, . . . , di, eo, . . . , ei]                                                A = [PIRE.sub.1, . . . , PIRE.sub.j, ao, . . . , ai, bo, . . . , bi, co,       . . . , ci,                                                                    do, . . . , di, eo, . . . , ei]                                                 ##STR3##                                                                       ##STR4##                                                                      for example:                                                                    ##STR5##                                                                      (the symmetry of revolution is imposed by choosing bi = 0)                     or:                                                                             ##STR6##                                                                      (in this case imposed symmetry of revolution)                                  or                                                                              ##STR7##                                                                      The gain function may also be broken down into Bessel                          functions.                                                                     Determination of the gain function                                              ##STR8##                                                                      F(A; θ.sub.j,n ; φ.sub.j,n) = Gr(θ.sub.j,n ;                   φ.sub.j,n) × PIRE.sub.j                                              Least squares criterion                                                        A = A such that the sum of the squares of the residues                          ##STR9##                                                                      Constraints                                                                    * PIRE.sub.j ε [PIRE.sub.min, PIRE.sub.max ]                           * φ.sub.j,n ε [0, θ.sub.max ]                                * θ.sub.j,n ε [0, 2]                                             * Gr(θ, φ) - Gr(θ+ Δθ, φ + Δφ)       ≦ ΔGr(Δθ, Δφ) (continuity                   constraint)                                                                    Δθ and Δφ: angular variation values                      ΔGr: maximum tolerable gain variation, for angular                       variations Δθ and Δφ.                                    * Gr(θ.sub.1, φ) = Gr(θ.sub.2, φ) (symmetry of             revolution constraint,                                                         optional) regardless of the values of θ.sub.1, θ.sub.2, and        φ                                                                          * Gr (θ.sub.0, φ.sub.0) = 0 for (θ.sub.0, φ.sub.0)         satisfying the equation                                                        H (θ.sub.0, φ.sub.0) = 0 (constraint of the main lobe                function,                                                                      optional), where H is the estimate for the equation of                         the boundary for the main lobe.                                                *Gr(θ, φ) ≧ 0 (positive gain constraint).                     *PIRE.sub.i = fixed PIRE value.                                                This constraint, which consists of fixing the value of                         one of the required PIRE values, can eliminate the                             ambiguity between the said PIREs values and the gain                           along the center line of the reception antenna.                                ______________________________________                                    

REFERENCES

[1] "Satellite Attitude From a Single GPS Antenna" by Y. Hashida and M. J Unwin (Proceedings of the Institute of Navigation (I.O.N) GPS 93 Technical Meeting, pages 355-363)

[2] "Using GPS to Determine the Attitude of a Spacecraft" by M. Martin-Neira and R. Lucas (March 1993, GPS world pages 49-54)

[3] "Use of Signal-To-Noise Ratio for Multipath Error Correction in GPS Differential Phase Measurements: Methodology and Experimental Results" by P. Axelrad, C. Comp and P. MacDoran (Proceedings of the Institute of Navigation (I.O.N) GPS 94, pages 655-666)

[4] "Markets Proliferating for Global Positioning Systems" by D. M. Graham (March 1990, "Sea Technology", pages 55-57)

[5] "Technical Description and Characteristics of Global Space Navigation System GLONASS-M" (International Telecommunication Union, Radiocommunication Study Groups, RTCA Papers, No. 50294/SC159-594

[6] "The GLONASS" by B. Panefieu (collection of transparencies used in a presentation made by the LRBA (Laboratoire de Recherche en Balistique en Aerodynamique--Aerodynamic Balistic Research Laboratory)

[7] "Accord de Standardisation; caracteristiques du systeme mondial de determination de la position NAVSTAR (GPS)" (Standardization Agreement; characteristics of the world system for determining the NAVSTAR position) (NATO, STANAG 4294). 

What is claimed is:
 1. Process for diagramming an antenna radiation pattern comprising the steps of:connecting an antenna to a receiver capable of supplying measurements of a C/No ratio, the C/No ratio being a ratio of useful signal power to noise power spectral density, and measurements of phase variation; placing the antenna on a support, pointing the antenna to signal source satellites in a constellation and receiving signals generated by the satellites in a constellation; wherein an antenna reception band is within a transmission band of the satellites; receiving the signals generated by the satellites and automatically recording the measurements of C/No ratio and phase variation corresponding to the signals generated by the satellites for a fixed period; and calculating the antenna radiation pattern diagram of the antenna from the measurements of C/No ratio and phase variation.
 2. Process according to claim 1, in which an antenna gain function is calculated by adjusting the measurements of C/No and phase variation using a least squares criterion.
 3. Process according to claim 1, in which the satellites in a constellation are not geostationary.
 4. Process according to claim 1, in which antenna gain is given by the formula ##EQU5## where θ and φ are spherical coordinates; ai, bi, ci, di and ei are coefficients.
 5. Process according to claim 1, in which the antenna is an antenna in the L,S,C, VHF or UHF band.
 6. Process according to claim 1, in which the satellites in a constellation are of the GPS, GLONASS, GLOBALSTAR, IRIDIUM or ORBCOMM type.
 7. Device for measuring the radiation pattern diagram of an antenna comprising:a support at the end of which an antenna to be measured is placed, the antenna being pointed to signal source satellites in a constellation, each satellite generating a signal; a receiver connected to the antenna to process the signals from the satellites, the receiver having a signal processor capable of supplying measurements of a C/No ratio, the C/No ratio being a ratio of useful signal power to noise power spectral density corresponding to the signal from the satellites; absorbent material placed on and around the support; and a calculating device which determines the antenna radiation pattern diagram of the antenna from the measurements of C/No ratio.
 8. Device according to claim 7, in which the antenna is placed under a radome.
 9. Device according to claim 7, comprising a pre-amplifier placed between the antenna and the receiver.
 10. Device according to claim 7, comprising an azimuth and elevation positioner on which the antenna is placed. 